A telecommunication cable or an optical cable has been used in communication between a numerical control device or the like as a main control unit and an interface device, and between a numerical control device or the like and a servo amplifier or the like as a drive control unit. In using either cable, a lead-around work of cables is troublesome and may cause disconnection thereof, and in particular the telecommunication cable may pick up an external electrical noise which may cause an abnormality in communication data.